This invention relates to electric machines with superconducting windings in general and more particularly to an arrangement for cooling a superconducting field winding in the rotor of an electric machine.
An arrangement for cooling the superconducting field winding of a rotor of an electric machine, especially a turbo-generator, with at least one coolant chamber which contains, in the operating condition, a vaporous and a liquid phase of a cryogenic coolant is known. The coolant is fed at a connecting head under subcritical pressure into a corotating feed line which is connected to the coolant chamber. Coolant paths extend through the field winding and are connected to the liquid space of the coolant chamber occupied by the liquid phase. At least one coolant discharge line is connected to the vapor space of the coolant chamber occupied by the vaporous phase. There is at least one coolant supply chamber containing liquid coolant. At least one further coolant path, which is arranged in loop-fashion, is thermally connected to a damper shield arranged around the field winding and is connected at its start and end to the coolant supply chamber. Such a cooling arrangement is described in the report by M. T. Brown et al. entitled "Rotor Cooling System for a 10-MVA Superconducting Generator" from Proceedings of the 1979 Cryogenic Engineering Conference, Madison, Wis, U.S.A., Aug. 21 to 24, 1979, Paper IC 9.
The superconducting field winding of a generator must be kept at a sufficiently low temperature during the operation of the machine that its superconductors do not undergo a transition into the normally conducting state. In general, cooling with liquid helium as the cryogenic coolant is therefore provided, and the dissipation heat produced in the superconductors and the heat introduced from the outside into the torque-transmitting parts of the rotor body lead to partial evaporation of the coolant. The heat flux introduced into the low temperature region of the rotor, however, can be greatly reduced, according to the principle of counterflow cooling, if the evaporated but still cold coolant exhaust gas is passed along the torque-transmitting rotor parts in good thermal contact, absorbs from these parts a large part of the inflowing heat, is warmed up in the process and leaves the rotor as warm gas at a connecting head.
In the superconducting field winding itself, dissipation heat is produced, among other things, if the superconductors are subjected to an alternating magnetic field. This takes place in the rotor, for instance, if, in the case of unbalanced load, not all current phases of the stator winding are loaded uniformly, and also after a short circuit switching sequence or in the case of oscillations when the rotor speed temporarily does not agree with the predetermined network frequency. In order to keep the alternating field amplitudes in the superconductors small in such transient operating states, the field winding may be surrounded by one or more damper shields which shield the magnetic field and at the same time damp oscillations. For shielding against low frequency fields, damper shields of thermally and electrically highly conductive material, kept at low temperature, are advantageous. For, during steady state generator operation, practicaly no dissipation heat is produced in a damper shield. A large amount of dissipation heat is temporarily released only during the relatively brief transient operating states, especially in the case of oscillations. Contrary to the superconductors of the field winding, the temperature of which must remain below the transistion temperature of its superconductive material even during the transient operating states, the temperature of the cold damper shield is allowed to rise higher, for instance, to 20 to 30K.
A turbo-generator with a field winding surrounded by such a damper shield is known from the report of the Electric Power Research Institute U.S.A.: "ERPI EL-577, Project 429-1, Final Report", November 1977, pages 3-258 to 3-270. In cooling the field winding, a so-called self-pumping effect is utilized, as is described, for instance, in the dissertation of A. Bejan: "Improved Thermal Design of Cryogenic Cooling Systems for a Superconducting Synchronous Generator", Ph.D Thesis, Massachusetts Institute of Technology (U.S.A.), December 1974, pages 148 to 159. The damper shield of the machine known from the ERPI report is fastened to a support cylinder of the rotor body, in which individual canals are provided which serve as damper cooling canals. For this indirect cooling of the damper shield, liquid helium is provided which flows in these canals due to a co-called thermo-siphon effect. Heat produced in the cold damper shield penetrates the support cylinder, gets into the helium contained in its cooling canals and starts convection there. Due to this convection, the warmed helium flows back into a helium bath present in the center of the rotor, where a corresponding amount of liquid helium evaporates.
In this known cooling arrangement it is assured that the heat from the cold damper shield is not transported directly into the area of the superconducting field winding since the damper cooling canals are thermally insulated from the winding cooling canals. In addition, good heat transfer from the support cylinder to the helium contained in its cooling canals makes rapid cooling of the damper possible. In this cooling arrangement, however, removal of generated gaseous helium is done using exhaust gas lines with a relatively small flow cross section. These exhaust gas lines run along the torque-transmitting parts of the rotor body. A large flow of helium vapor which occurs suddenly in the case of a disturbance then cannot leave the rotor fast enough, so that the pressure in the central helium bath rises accordingly. A consequence thereof is also an increase of the saturation temperature of the helium of this bath, used for cooling the superconducting field winding, and therefore, of the temperature of the winding itself.
These difficulties are largely avoided in the above-mentioned cooling arrangement for a corresponding turbo-generator known from the report from the "Proceedings of the 1979 Cryogenic Engineering Conference". For, in this cooling arrangement, the cooling of two damper shields and the winding cooling are largely separated from each other. Accordingly, there is provided, in addition to a central coolant chamber for receiving the coolant required for cooling the superconducting field winding, a separate cooling supply chamber in which the liquid helium required for cooling the damper shields is stored. In addition, a phase separator for separating gaseous helium components is provided in the coolant loop for cooling the damper shield. Now, if heat is suddenly released in the damper shields, a coolant circulation is developed, due to a thermo-siphon effect, from the helium bath contained in the coolant supply chamber through the first damper shield to a phase separator and through the second damper shield back to the helium bath. Due to the supply of heat, helium vapor is then generated in the helium bath of the coolant chamber, and in the phase separator; this helium vapor can escape through special gas outlets and further, through an emergency outlet provided in the separator. A return valve in the connection between the coolant supply chamber and the coolant space containing the coolant bath for the winding prevents any reaction of the pressure rise in the coolant supply chamber on the coolant bath of the winding. This valve also makes it possible to fill the coolant supply chamber with liquid helium. However, a difficulty still exists with this cooling arrangement, in that the helium vapor, which is formed in the coolant bath of the winding, cannot escape as long as the return valve of the coolant supply chamber is closed. Then, the pressure and therefore, the temperature of the coolant provided for cooling the field winding, can rise however. In addition, proper operation of the return valve with its moving parts can be assured at low temperatures only at relatively high cost.